Rot13 with Python

#!/usr/bin/python3
import sys, string
def rot13(s):
    return ''.join([chr(x.islower() and ((ord(x) - 84) % 26) + 97
                        or x.isupper() and ((ord(x) - 52) % 26) + 65
                        or ord(x))
                    for x in s])
for line in sys.stdin:
    sys.stdout.write(rot13(line))